Lab 4 – Sea Floor Changes in a Volcanically Active Setting
Instructor Guide

This lab is meant to give students an opportunity to apply and extend their introductory knowledge about plate tectonics to make sense of what goes on inside a magma chamber before, during, and after an eruption. 

Approximate time involved: approximately two hours total.

Learning outcomes

  • LO1. Demonstrate skills working with data that include: distinguishing between raw and processed data; identifying relevant data to answer questions; reading multi-axes graphs.
  • LO2. Explain how water depth data are calculated from water pressure data.
  • LO3. Describe the possible relationship between earthquakes and seafloor topography changes.
  • LO4. Determine what earthquake & bathymetry data tell us about processes within a volcano.
Learning outcome Activity 1 Activity 2 Activity 3
Outcome 1 introduced guided practice guided practice
Outcome 2 introduced/guided practice
Outcome 3 introduced/guided practice independent practice
Outcome 4 introduced/guided practice guided practice

Materials needed: simple calculator; computer or similar device; internet access

What students should know before this activity

  • Data knowledge: basic graph reading abilities
  • Content knowledge: plate tectonics, volcanic processes, sea floor features

What instructors should know before this activity

Optional pre-lab activities:

Prior to this lab, it would be worthwhile to introduce background about why volcanoes erupt, what happens during an eruption, and how calderas form.  It is recommended that students are guided to understand the basics about processes at plate boundaries through some sort of engaging exercise in addition to any lecture provided on this topic.

Scientific Background


Web resources: seafloor features, earthquakes, plate tectonics, volcano formation and sources of magma.

Introductory Geology OER textbooks:

  • Physical Geology by S. Earle (2019): Chapter 4 (Volcanism) and Chapter 11 (Earthquakes)
  • An Introduction to Geology by C. Johnson et. al. (2017): Chapter 4.3 (Magma Generation), Chapter 4.5 (Volcanism), Chapter 9.6 (Earthquake Essentials)

Teaching notes

Intro page:

  1. Review where all the devices are located around Axial Seamount. What kind of data will be collected from them?
    • The bulleted lists in Figure 1 provide quite a bit of information for students to answer this question. The purpose of some instruments might not be obvious to students but the point of this question is for students to get a feel for how intensively studied this seamount is. There are over 20 instruments at the site and is considered the “most advanced volcanic observatory in the worlds’ oceans” ( Note that the North arrow in Fig. 1b is easy to miss and located in the lower right corner.  Also, the colorations for this figure were enhanced a bit to highlight the features.  Use the map in Lab 4.1 for a more accurate representation of the topography of the seafloor.  The location of lava fields in the caldera can be seen in the map at
  2. Define the key terms vent field and caldera. Students can click on the link to view definitions:
    • vent field – Concentrated occurrences of fissures in the seafloor that emit heated mineral-rich fluids and sometimes form black and white smokers where, commonly occurring in volcanically active areas on the seafloor.
    • caldera – Depression in the center of an active volcano.

Activity 4.1:

The map provided highlights the location of some of the seamounts in the Cobb-Eickelberg Seamount Chain.  At the southeast end of the chain is Axial Seamount, the feature that is monitored by the numerous devices illustrated in the previous page.  Zoom in and out of the map to develop familiarity with Axial Seamount and other seamounts in the chain, as well as nearby features.  Note that Axial Seamount does not have a prominent cone shape that many people associate with volcanoes, which is characteristic for many “shield volcanoes” mentioned in the previous page.  The pin for Axial Seamount in this map is positioned inside the caldera in the general vicinity of the numerous instruments used to collect data included in this lab.

  • Students should be able to identify the seamounts in the Cobb-Eickelberg Seamount Chain and note that the shape of Axial Seamount is a bit different from others in the area: larger, less symmetrically shaped; larger caldera.
  • Students put into words differences and similarities they notice, as well as location on sea floor relative to other features they might know about or notice in the map (e.g. Juan de Fuca Ridge).  Unless you prefer full sentences and a coherently constructed paragraph, bullets could be used in answering.

Questions 2-3 asks students to compute unit conversions to put size in context of units they might be more familiar with. They might need help setting up an equation that can be used to convert any measurement.  Encourage students to set up an equation rather than using an online unit conversion calculator. And instructors might compare the size of Axial Seamount to a structure students might be familiar with given their unique perspectives.

The interactive time series graph displays pressure measured at the seafloor, within the Axial Seamount caldera, and depth below the surface calculated from the seafloor pressure data (with tides removed).

  • Point out the note below the graph: The coding for this interactive widget has a quirky feature that somehow causes the y-axis to flip when certain zooming actions are implemented.  Until this issue is resolved, the reset is easy.

Question 4 asks students to estimate water depth from pressure at one point in time, using the relationship that water pressure increases about 15 psi (pound-force per square inch) for every 10 meters (33 feet) of depth. This is likely to be difficult for some students…help them think through that the ratio of between depth and pressure is important and can be calculated from a point on the graph, then used as the means to convert any pressure reading. There is a hint with an example calculation below this question. Instruct students to use a different date/time for their own calculation.

    • The Bigger picture: The volume of water at each depth in the water column exerts pressure downwards; the deeper the water column, the greater the pressure. Pressure increases about 15 psi (pound-force per square inch) for every 10 meters (33 feet) of depth. Thus a pressure of 2250 psi equates to 1500 meters of depth [2250 psi ÷ (15 psi/10m) = 2250 psi ÷ 1.5 psi/m = 1500 m depth].

Questions 5-6 ask students to speculate on the cyclical pattern in the pressure data. The “raw” pressure data show an unrelated phenomenon that turns out to be an excellent example of mixed semidiurnal tides. Because tides typically come sometime after geology topics in a typical oceanography class, this step and question is meant merely to observe the “raw” data and review some interesting information that they are not expected to explain at this point.  Below are some extension questions you could revisit when tides are addressed if desired:

  • What challenges might they present for oceanographers when they want to study average water depth at this site
  • When multiple processes occur at the same time, their impact on a particular feature (pressure in this case) can cause the data to be complicated and “messy”.  Scientists need to process messy data somehow when possible.

Question 7-9 focus on the processed seawater depth data, which show a step change on April 24, 2015. This step is meant to help guide students to develop their data patterns description skills; the authors have noticed that it is a skill needing some guidance for most students particularly in an intro class.  Without extensive experience, students may not understand that thorough data descriptions are needed first in order to draw conclusions, and some descriptions are then needed as evidence to support conclusions in a complete explanation. If possible, discuss the answers with students to reveal some of their preliminary thinking and allow them to consider other students’ ideas.

The final question (#10) in this activity is an important recap and transition to Activity 4.2

    • (a) is an exercise in being really thorough in describing trends and patterns.  Help students include use of specific data as evidence of the patterns they describe.
    • (b) is a step that asks students to refine some of their ideas without formalizing an answer yet.  If a discussion of some sort is possible, it might help students in this step by considering each others’ ideas.


Activity 4.2

  1. Describe the patterns in each and compare patterns in both graphs and to the water depth plot.
    • This is a next step exercise to thoroughly describe the data. Students should describe trends and patterns in each individual set and across data sets. They may start by noting general patterns; encourage them to include both general and very specific patterns.
  2. Considering the combination of seafloor depth and earthquake data, what do you think happened at Axial Seamount on April 24th, 2015?
    • If possible, allow students to discuss before moving on.
  3. What do you think might explain the difference between the number of earthquakes before and after the eruption?
    • Discuss to collect students’ ideas; encourage their use of relevant science concepts previously addressed.  Don’t underestimate their ability to be creative and propose some components of a correct response (addressed in next question).
  4. Discuss with your classmates and instructor in class or in a discussion board how the movement of magma in the subsurface within the volcano might influence the behavior of the earthquakes before, during, and after the 2015 event? Also, how might the movement of the magma influence the depth of the seafloor?  If you are struggling still to picture what takes place beneath the surface as magma accumulates before an eruption, watch this video or changes to Mount Etna over time; or this series of images about what occurs in a magma chamber and caldera formation; or this site about the August 2015 research cruise to Axial Seamount and how research goals were met.
    • Ask students to propose an explanation, then with their ideas, discuss to help them use background supplied either in the lab document, other resources supplied, or in your class to develop an appropriate explanation. Instructors can review the linked resources ahead of time to prepare to guide this discussion.
  5. After discussing in class, write your own explanation for the trends and patterns in the data.
    • This is the formal step where students can be evaluated on their ability to support a conclusion with a thorough explanation that includes appropriate and sufficient evidence from their data descriptions and their understanding of important relevant science concepts.

Optional Extension Exercise (revisit after studying tides!)

  1. Adjust the widget to show pressure data for the time span that includes only March 15 through March 22. Describe the pattern(s) you see in each of the graphs.
    • The earthquake data are very messy, but patterns are present in the number of earthquakes compared to the seafloor pressure associated with tides. If this step is done interactively with students, they likely would benefit from some guidance like the prompts included in 4.8 question 8 and 4.2 question 1.
  2. What do you think might explain the patterns you’ve noted? What background from this lab might be useful in trying to explain them?  Take a look at Wilcock et al. (2016) for their ideas about what might be happening.
    • This is a bit tricky to answer and in fact scientists are trying to figure this out!  However, if students consider ALL possible data sets provided in Lab 4, they may propose what scientists have hypothesized!

Source: Wilcock, W.S.D, Tolstoy, M., Waldhauser, F., Garcia, C., Tan, Y.J., Bohnenstiehl, D.R., Caplan-Auerbach, J., Dziak, R.P., Arnulf, A.F. and Mann, M.E., 2016, Seismic constraints on caldera dynamics from the 2015 Axial Seamount eruption, Science 354 (6318): 1395-1399.  [doi: 10.1126/science.aah5563]


Activity 4.3

The objectives of this activity are to have students articulate a conceptual model of subsurface magma movement and the associated changes to the seafloor within the seamount caldera, using evidence from data examined in activities 4.1 and 4.2.

  • Question 1: Help students to be as thorough as possible in recording details for activities they can observe through the entire video.
  • Question 2: Discuss ideas with students and add details to fill in the gaps.  Students will individually write their own complete explanation next.
  • Question 3: Students should individually compose a complete explanation to be assessed for the presence of use of evidence and their understanding of relevant science concepts.
  • Question 4: Depending on time available, students’ questions could be discussed particularly to clear up misunderstandings or gaps in understandings.  Guide students to consider questions that would extend their learning beyond Lab 4.  If time allows, students could attempt to research answers, or such work could be assigned as homework, etc.


Additional Resources

Additional seafloor bathymetry images:

Open source introductory texts for additional geology background:



Arnule et al., Axial 3D Seismic Expedition 2019,

Chadwell, W.W., Butterfield, D.A., Embley, R.W., Tunnicliffe, V., Huber, J.A., Nooner, S.L., Clague, D.A., Spotlight 1: Axial Seamount (PDF). Oceanography. 23 (1): 38-39. Retrieved 14 August 2020.

Chadwick, J., Keller, R., Kamenov, G., Yogodzinski, G., Lupton, J., 2014, The Cobb hot spot: HIMU‐DMM mixing and melting controlled by a progressively thinning lithospheric lid, Geochemistry, Geophysics, Geosystems 15 (8): 3107-3122.

Discovery News, 2009, Undersea Volcano Eruptions Caught On Video,

ESA/NASA/JPL-Caltech, 2012, PIA13201: Mount Etna InSAR Time Series Animation, NASA Jet Propulsion Lab Photojournal, [Video also available at]

Gudmundsson, A., 2013, Magma-chamber geometry, fluid transport, local stresses, and rock behaviour during collapse-caldera formation.

Leifert, H, 2016, An undersea volcano yields its secrets, Earth,

MBARI, Submarine Volcanoes Group

Mittelstaedt, E., Fornari, D.J., Crone, T.J., Kinsey, J., Kelley, D., Elend, M., 2016, Diffuse venting at the ASHES hydrothermal field: Heat flux and tidally modulated flow variability derived from in situ time‐series measurements, Geochemistry, Geophysics, Geosystems 17(4): 1435-1453. [For additional consideration of tidally influenced EQ activity]

Mount St. Helens Science and Learning Center, ND, Volcano Deformation Student Worksheet,

NOAA Axial Seamount information:

NOAA, 2009, Deep Ocean Volcanoes,

NSF, Underwater volcano’s fiery eruption captured in detail by seafloor observatory, News Release 16-152.

OOI staff. 2015. Axial Eruption Site Identified – Bathymetric Survey Data Available Online.

OOI staff. 2015. OOI Team First to See April 24, 2015 Eruption of Axial Seamount.

OOI staff, 2020. From Whale Songs to Volcanic Eruptions: OOI’s Cable Hears the Sounds of the Ocean.

Pacific Island Ranger, 2010, Inflation and Deflation of Hawaiian Volcanoes, accessed 5/20/2021;

Rubin, K.H., S.A. Soule, W.W. Chadwick Jr., D.J. Fornari, D.A. Clague, R.W. Embley, E.T. Baker, M.R. Perfit, D.W. Caress, and R.P. Dziak. 2012. Volcanic eruptions in the deep sea. Oceanography 25(1):142–157,

Smith, L., 2018, Community Tools available on the OOI website to examine the Axial Volcano;

Smithsonian Institution, Global Volcanism Program: Axial Seamount,

Wilcock, W.S.D, Tolstoy, M., Waldhauser, F., Garcia, C., Tan, Y.J., Bohnenstiehl, D.R., Caplan-Auerbach, J., Dziak, R.P., Arnulf, A.F. and Mann, M.E., 2016, Seismic constraints on caldera dynamics from the 2015 Axial Seamount eruption, Science 354 (6318): 1395-1399.  [doi: 10.1126/science.aah5563]

Wilcock, W.S.D, Dziak, R.P., Tolstoy, M., Chadwick Jr., W.W., Nooner, S.L, Bohnenstiehl, D.R., Caplan-Auerbach, J., Waldhauser, F., Arnulf, A.F., Baillard, C., Lau, T., J.H., Tan, Y.J., Garcia, C., Levy, S., and Mann, M.E., 2018, The recent volcanic history of Axial Seamount, Geophysical Insights into Past Eruption Dynamics with an Eye Toward Enhanced Observations of Future Eruptions, Oceanography Vol.31(1): 114-123.